Remineralization is a natural repair process for cari- ous lesions. Elevated levels of fluoride in toothpaste have been shown to be effective in rehardening root caries lesions that are cavitated. A 5000-ppm-F tooth- paste rehardened 76% of lesions compared with 35%
in a 1100-ppm-F group. The concept of the caries balance proposed by Featherstone describes three pathological factors and three protective factors for dental caries. The pathological factors are (1) acid- producing bacteria, (2) frequent consumption of fermentable carbohydrates, and (3) below normal salivary flow and function. The three protective fac- tors are (1) a normal salivary flow and components, (2) fluoride, and (3) antibacterials. Two salivary com- ponents required for remineralization are calcium and phosphate. Fluoride enhances remineralization.
Calcium phosphate formulations have been developed for addition to toothpaste, varnishes, and gum (Fig. 8.11). A calcium phosphate remineraliza- tion technology based on casein phosphopeptide- amorphous calcium phosphate (CPP-ACP) was effective in remineralizing enamel subsurface lesions by stabilizing high levels of calcium and phosphate
ions. When added to sugar-free gum in a random- ized controlled clinical trial, an 18% reduction in car- ies progression after 24 months was demonstrated.
In paste form, the CPP-ACP complexes have been shown to be effective in reversing early caries lesions and stabilizing the progression of caries.
A bioactive glass (calcium sodium phosphosili- cate) originally developed as a bone-regenerative material has been shown to deposit onto dentin surfaces and mechanically occlude dentinal tubules when delivered in a dentifrice. When combined with therapeutic levels of fluoride, this material increases the remineralization of caries lesions in situ.
There are many new calcium silicate–based mate- rials that have been developed for tooth lining with the main objectives being the protection of the pulp and the potential remineralization of overlying den- tin. Many of these materials are light-cured and therefore have reasonable mechanical properties and solubility resistance. They are the subject of many research studies, specifically looking at the effect the materials have on nearby odontoblast cells, which may be stimulated to form extracellular matrix as the initiation of the mineralization process, or to cause undifferentiated cells to differentiate into odonto- blast-like cells. The ultimate success of such materi- als and strategies is not yet known.
One final material that also has been suggested to cause tooth remineralization, especially of frank car- ies lesions, as well as having a strong antibacterial effect is silver diamine fluoride. Silver diamine fluo- ride (SDF) is solution of about 30% silver diamine fluoride (of which approximately 25% is silver and
5% is fluoride) in water with a pH of approximately 10. The high pH makes it a strong neutralizer of acids, but perhaps more important is its strong anti- bacterial effect, predominantly due to the presence of silver, a known antimicrobial. The material was preceded by silver nitrate used in a similar way, that is, the application of the colorless liquid to frank car- ies. There is growing evidence that caries is arrested in the presence of the material, and this has great advantages for use in children and geriatric popula- tions especially. The problem with the material is that it stains tooth structure, as well as other objects such as skin or clothing. Therefore while it may be ben- eficial for arresting lesions, it requires being covered by an esthetic material to block out its black- staining effect on the tooth. The material was originally approved as a dentin desensitizer, but it is acceptable now to use it for arresting caries.
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135 The concept of a composite biomaterial, introduced in Chapter 4, can be described as a solid that contains two or more distinct constituent materials or phases when considered at greater than an atomic scale. In these materials, mechanical properties such as elastic modulus are significantly altered in comparison with a homogenous material consisting of either of the phases alone. Enamel, dentin, bone, and reinforced polymers are considered composites, but alloys such as brass are not. The ability to change properties of the macroscale object based on control of the indi- vidual constituents is a significant advantage for the use of composite materials. In dentistry, the term
“resin composite” generally refers to a reinforced polymer system used for restoring hard tissues, such as enamel and dentin. The proper materials science term is “polymer matrix composite” or for those composites with filler particles often used as direct- placed restorative composites, “particulate-rein- forced polymer matrix composite.” In this chapter, the term “resin composite” will refer to the reinforced polymer matrix materials used as restorative materi- als. The class of materials called conventional glass ionomers (GIs) and resin-modified glass ionomers (RMGIs) also fall in the scientific class of composite materials but because these are water-based mate- rials and have a distinct acid-base setting reaction, they have been traditionally categorized as a class of their own. It is important to remember, however, that most biological materials including enamel, dentin, bone, connective tissue, muscle, and even cells are classified as composites within the broad range of biological engineering materials.
Resin composites are used to replace missing tooth structure and modify tooth color and contour, thus enhancing esthetics. A number of commercial resin composites are available for various applications.
Traditionally, some have been optimized for esthetics and others were designed for higher stress bearing areas. More recently, nanocomposites have become
available, which are optimized for both excellent esthetics and high mechanical properties for stress bearing areas. GIs and RMGIs are used selectively as filling materials usually for small lesions, especially where one or more margins are in dentin and in areas of caries activity.
Resin-based composites were first developed in the early 1960s and provided materials with higher mechanical properties than acrylics and silicates, lower thermal coefficient of expansion, lower dimen- sional change on setting, and higher resistance to wear, thereby improving clinical performance. Early composites were chemically activated followed by photoactivated composites initiated with ultravio- let (UV) wavelengths. These were later replaced by composites activated in the visible wavelengths.
Continued improvements in composite technology have resulted in the modern materials with excellent durability, wear resistance, and esthetics mimicking the natural teeth. In particular, the incorporation of nanotechnology in controlling the filler architecture has made dramatic improvements in these materials.
Moreover, the development of bonding agents for bonding composites to tooth structure (see Chapter 13) has also improved the longevity and perfor- mance of composite restorations.
A classification of preparation type and recom- mended composite category is listed in Table 9.1.
Characteristics of these composite categories are summarized in Table 9.2.
GIs were developed in the 1960s and are based on an acid-base cement-forming reaction between fluoroaluminosilicate (FAS) glass powder similar to those used in silicate cements and aqueous solution of polycarboxylic acids. These were less prone to dis- solution than silicates but the early materials suffered from difficulty of manipulation, technique sensitivity, and poor esthetics. Advances in these materials have continued and the modern materials have improved properties. RMGIs were invented in the late 1980s to
9
Restorative Materials: Resin Composites
and Polymers
preserve the advantages of fluoride release and clini- cal adhesion of the conventional GIs yet provide the ease of light curing and good esthetics of resin-based materials. The use of nanotechnology in RMGI has resulted in enhanced esthetics of these materials.